1. Field of the Invention
The present invention relates generally to apparatus for measuring respiratory pressure or flow and, more specifically, to respiratory pressure transducers. More particularly, the present invention relates to pressure transducers which may be positioned proximate to a respiratory conduit which is configured to communicate with the airway of an individual.
2. Background of Related Art
Respiratory flow measurement during the administration of anesthesia, in intensive care environments, and in monitoring the physical condition of athletes and other individuals prior to and during the course of training programs and other medical tests provides valuable information for assessment of cardiopulmonary function and breathing circuit integrity. Many different technologies have been applied to create a flow meter that attempts to meet the demanding requirements of these environments.
Although various other types of pressure measurement apparatus are known, differential pressure flow meters have conventionally been used to obtain respiratory flow measurements. While pressure monitoring is typically performed to measure delivered (i.e., inspired) and exhaled volume by monitoring respiratory mechanics parameters, such as airway pressures, flow rates, and breath volumes, clinicians can better provide quality health care to patients requiring breathing assistance. Additionally, pressure monitoring may be used in conjunction with respiratory gas measurements to assess other respiratory parameters, such as oxygen consumption, carbon dioxide elimination, and even cardiac output or pulmonary capillary blood flow.
Differential pressure flow meters operate on the basis of Bernoulli's principle: the pressure drop across a restriction is proportional to the volumetric flow rate of the air. The relationship between flow and the pressure drop across a restriction or other resistance to flow is dependent upon the design of the resistance. In some differential pressure flow meters, which are commonly termed “pneumotachs”, the flow restriction has been designed to create a linear relationship between flow and a pressure differential. Such designs include the Fleisch pneumotach, in which the restriction is comprised of many small tubes or a fine screen to ensure laminar flow and a more linear response to flow. Another physical configuration is a flow restriction having an orifice that varies in relation to the flow. Such designs include the variable orifice, fixed orifice and venturi-type flow meters. Exemplary patents for variable orifice differential pressure flow sensors include U.S. Pat. No. 4,993,269, issued to Guillaume et al. on Feb. 19, 1991, U.S. Pat. No. 5,038,621, issued to Stupecky on Aug. 13, 1991, U.S. Pat. No. 5,763,792, issued to Kullik on Jun. 9, 1998, and U.S. Pat. No. 5,970,801, issued to Ciobanu on Oct. 26, 1999. Exemplary patents for fixed orifice differential pressure flow sensors include U.S. Pat. No. 5,379,650, issued to Kofoed et al. on Jan. 10, 1995, U.S. Pat. No. 5,925,831, issued to Storsved on Jul. 20, 1999, and U.S. Pat. No. 6,203,502, issued to Hilgendorf on Mar. 20, 2001.
Many known differential pressure flow sensors suffer deficiencies when exposed to less than ideal gas and flow inlet conditions and, further, possess inherent design problems with respect to their ability to sense differential pressure in a meaningful, accurate, repeatable manner over a substantially dynamic flow range. This is particularly true when the flow sensor is needed to reliably and accurately measure low flow rates, such as the respiratory flow rates of infants. Proximal flow measured at the patient's airway can be substantially different from flow measured inside or at the ventilator. Many ventilators measure flow, not at the proximal airway, but close to the ventilator. Measurements of flow in this way may result in a substantial difference between the flow, pressure, and volume of gases that are delivered to or exhaled by the patient and that are reported by a pressure or flow sensor which is associated with the ventilator. At least a portion of this discrepancy is because of wasted compression volume, which distends and may elongate a length of respiratory conduit positioned between the patient's airway and the pressure or flow sensor, and humidification or dehumidification attributable to the length of the respiratory conduit between the patient's airway and the pressure or flow sensor. As the compliance of the respiratory conduit may be a known value, some ventilator manufacturers apply a correction for the wasted compression volume. Even when a correction is applied, precise estimation of the wasted and inhaled portions of the compression volume is difficult because of variations between individual respiratory conduits, the use of humidifiers, the use of heat-moisture exchangers, and other circuit components. Within a typical respiratory conduit, gas conditions (e.g., temperature, pressure, humidity, etc.) may vary considerably, depending upon the distance of the gases from the airway of the monitored individual. As gas conditions nearest the individual are most likely to reflect the corresponding conditions within the individual's airway, the preferred location for monitoring inspiratory and expiratory flows from a patient in the critical care environment is proximal (i.e., as close to the individual's airway as possible).
Routine clinical use of differential flow meters has increased significantly in the last few years with the development of more robust designs, such as that disclosed in U.S. Pat. No. 5,379,650, issued to Kofoed et al. on Jan. 10, 1995 (hereinafter “the '650 Patent”), the disclosure of which is hereby incorporated herein in its entirety by this reference. The differential flow meter described in the '650 Patent, which has overcome the majority of the problems that were previously encountered when prior differential pressure flow sensors were used, includes a tubular housing containing a diametrically oriented, longitudinally extending strut. The strut of the flow sensor disclosed in the '650 Patent includes first and second lumens with longitudinally-spaced pressure ports that open into respective axially located notches formed at each end of the strut.
Despite such improvements in the performance of differential pressure flow meters, differential pressure flow meters continue to include a pneumotach positioned along a respiratory conduit, a typically remotely positioned pressure monitor, and tubing that operatively connects the pneumotach and the pressure monitor with one another. The tubing transmits pressure at each port of the pneumotach to one or more pressure sensors that are contained within the monitor. Typically, several feet of flexible, small bore, dual or triple lumen tubing are used to connect the pneumotach and the pressure monitor to one another.
The use of such small bore tubing is, however, somewhat undesirable from the standpoint that the pressure samples which are conveyed from the pneumotach may be damped or distorted as they travel through the tubing. Consequently, it is often necessary to screen the sensors and individually balance the internal pneumatics of the monitor to ensure accurate measurement of airway pressure and, thus, to provide an acceptable level of clinical performance under conditions such as those typically encountered with monitored patients (i.e., low ventilatory compliance).
Additionally, the use of flow sensors with tubing in clinical environments, such as critical and intensive care where high humidity is often the norm, leads to the condensation of moisture in the pressure transmission tubing, whether or not the pressure transmission tubing or any portion of the respiratory conduit is heated. One result of condensation is a damping and distortion of respiratory samples, or the pressure “signals”, that propagate down the pressure transmission tubes. Typically, pressure transmission tubes are periodically purged with air from a compressed gas source or a pump in order to reduce the adverse effects of condensate on pressure and flow measurements, which creates additional work for healthcare personnel and, therefore, is a somewhat undesirable practice. Additionally, accidental or intentional disconnection of the tubing from the monitor may cause condensate or even sputum to flow into the pressure transmission tubes and potentially contaminate the monitor.
Further, the lengthy tubing of conventional differential pressure flow sensors is typically discarded after use, resulting in a significant amount of plastic waste. Accordingly, employing such tubing as a single-use element of a differential pressure flow sensor is costly.
Reuse of pressure transmission tubing is also typically contrary to manufacturer recommendations due to the potential for contamination of the pressure transducer, as well as the potential (although not significant) extubation hazard posed thereby in the clinical environment. Nonetheless, such tubing is often reused, particularly outside of the United States, as a cost-saving measure.
The inventors are not aware of a differential pressure flow sensor that lacks pressure transmission tubes extending between the pneumotach and monitor thereof, or of a differential flow sensor that includes a transducer, that is configured to be carried upon a respiratory conduit that communicates with the airway of an individual.